Turn FollowMobj into a mobj hook

Previously it was considered a player hook, which means that making it run for a specific mobj type would need to be done manually in the hook. Since FollowMobj is clearly intended for custom characters with their own follow item objects, and wanting it to run for everything is far less likely, this is a bit silly.

This should be backwards compatible with everything; anything this would technically "break" (rather, retroactively fix) are scripts that already assumed the hook had a type parameter and didn't put any checks.